It is quite customary in the engine diagnostic art to employ a cathode ray tube and to vertically shift the position of the horizontal trace in accordance with the voltage appearing across the plugs. This voltage is indicative of various conditions such as compression, condition of the plug, etc. In recent years, the time during which a voltage pulse is applied to the plug has become extremely short. This has been particularly true in connection with capacitor discharge types of ignition. It is obviously desirable that the ignition take place as quickly as possible and with a capacitor discharge type of system, this becomes possible. The problem, however, that is introduced when the length of the voltage pulse applied to the plug is very short is that it is difficult for the cathode ray tube to respond to this extremely short pulse. Even if such features as a trace brightener circuit are added, it is still very hard to get an accurate indication of the height of the pulse. This is particularly true when it is considered that these oscilloscopes are often used in relatively dirty locations where the screen tends to get covered with a film of grease. Furthermore, the lighting is often not too desirable for proper reading of a cathode ray tube.